1. Field of the Invention
The invention relates to a photoelectric conversion device, and more particularly to a photoelectric conversion device that uses a wavelength converting mechanism.
2. Description of Related Art
Solar cells capable of converting sunlight energy directly into electricity are expected to serve as a next-generation source of green energy. Because there is a certain limitation to the area for installing a solar cell, the photoelectric conversion efficiency must be increased to obtain a greater amount of electricity. To this end, current development efforts are aimed at, for example, optimizing the device structure and manufacturing operations, and achieving higher levels of quality in the silicon used as the primary material.
Technology relating to such solar cells includes, for example, Japanese Patent Application Publication No. 2009-59915 (JP-2009-59915 A) that discloses art relating to a “hot carrier-type” solar cell that, in order to reduce energy loss of charge carriers (electrons and holes; the same applies below) generated using light having a higher energy than the energy gap of the semiconductor making up a light absorbing layer, promotes energy interactions (transfer) between carriers in the light absorbing layer and extracts electrons having a high energy. Japanese Patent Application Publication No. 2004-296658 (JP-2004-296658 A) discloses art relating to a multi-junction solar cell that uses, as a top cell, a solar cell that is formed of an AlInGaP material and has a p-n junction, and uses, as a bottom cell, a solar cell that is lattice matched with the top cell, formed of an InGaAsN material and has a p-n junction, in which multi-junction solar cell the compositional ratio of aluminum in the group III elements of the AlInGaP material making up the top cell is in the range of 0.05 to 0.15. Japanese Patent Application Publication No. 2006-114815 (JP-2006-114815 A) discloses a solar cell that has a p-i-n structure and contains, in an i layer that is a photodetection layer, quantum dots that exhibit three-dimensional quantum confinement effects, wherein the quantum dots and the barrier layers enclosing them have a type II band structure. International Publication No. 2008/047427 (WO 2008/047427) discloses a solar cell module with a structure that incorporates a plurality of units having a front cover, a back cover and, between these covers, a crystalline silicon cell encapsulated within an encapsulant, wherein the encapsulant between the front cover and the crystalline silicon cell contains a fluorescent resin composition composed of an ethylene-vinyl acetate copolymer containing 0.01 to 10 wt % of an organic rare earth metal complex that fluoresces in the wavelength range of 550 to 900 nm. In addition, Solar Energy Materials and Solar Cells (Netherlands), Vol. 91, No. 9, 829-842 (2007) discloses art relating to an “up-conversion type” solar cell that reduces light transmission loss by long-wavelength light having lower energy than the energy gap of the semiconductor making up the light absorption layer by converting the long-wavelength light to a wavelength suitable for the energy gap of the semiconductor making up the light absorption layer.
To obtain a high photoelectric conversion efficiency in a hot carrier solar cell, it is necessary to promote energy interactions (transfer) between the charge carriers and allow the carriers to move from the absorption layer to the electrodes while retaining a high energy. Hence, it is thought to be essential for hot carriers to have a lifetime of at least one nanosecond. Yet, in current semiconductor materials, the lifetime of hot carriers is limited to a range of from several picoseconds up to several hundreds of picoseconds. Hence, even using the art disclosed in JP-2009-59915 A, the photoelectric conversion efficiency increasing effect has tended to be inadequate. In the multi-junction type solar cell disclosed in JP-2004-296658 A, because light of a broad range of wavelengths included in sunlight can be absorbed, it should presumably also be possible to increase the photoelectric conversion efficiency. However, in a multi-junction type solar cell, the number of semiconductor interfaces having a high density of defects that annihilate carriers and cause a decline in the photoelectric conversion efficiency rises due to the increased number of junctions. Also, costs have tended to rise on account of the need to use many expensive III-V compound materials and the increased number of production steps. In the up-conversion solar cell disclosed in Solar Energy Materials and Solar Cells (Netherlands), Vol. 91, No. 9, 829-842 (2007) and down-conversion solar cells that reduce energy loss by converting high-energy, short-wavelength light to a wavelength light suitable for the energy gap of the semiconductor making up the light absorbing layer, fluorescent materials that employ rare-earth elements such as those disclosed in WO 2008/047427 are used in many cases. However, because the wavelength range of light that can be absorbed by conventional fluorescent materials that employ rare-earth elements is narrow and energy loss during wavelength conversion is large, the photoelectric conversion efficiency-increasing effects of conventional down-conversion solar cells and up-conversion solar cells have tended to be inadequate. That is, even by combining the teachings of the various art disclosed in JP-2009-59915 A, JP-2004-296658 A, JP-2006-114815 A, WO 2008/047427 and Solar Energy Materials and Solar Cells (Netherlands), Vol. 91, No. 9, 829-842 (2007), it is difficult to increase the photoelectric conversion efficiency.